Project Summary This research effort seeks to increase fundamental knowledge about how cells handle failures in protein synthesis and how these failures can lead to disease. Protein synthesis by the ribosome can abnormally halt (?stall?) for numerous reasons, including faulty mRNA, insufficient availability of translation components, and genetic errors. Handling these failures is critical, as abnormal responses to stalled protein synthesis have been demonstrated to cause neurodegeneration. To cope with this burden, eukaryotic cells employ a process called Ribosome-associated Quality Control (RQC) that detects stalled ribosomes and promotes degradation of the protein that the ribosome was previously synthesizing (the ?stalled nascent chain?). As part of RQC, we discovered that the protein Rqc2 hijacks the ribosome to append Carboxy-terminal Alanine and Threonine residues (?CAT tails?) to the stalled nascent chain. CAT tails benefit cells by marking stalled nascent chains for degradation. Curiously, CAT tails can also harm cells by forming toxic aggregates that impair cellular viability. How CAT tails possess these seemingly contradictory behaviors (beneficial vs. toxic) remains unclear. This research program will address this knowledge gap by using biochemical, cell biological, and genomic approaches to study: 1) how cells synthesize CAT tails , 2) how cells degrade CAT tails, and 3) how CAT tails behave in metazoans. This study will elucidate a fundamental mechanism cells use to protect themselves from failed protein synthesis and uncover the causes and consequences of its misregulation.